The present invention relates to probe stations, commonly known as package or wafer probers, used manually, semiautomatically or fully automatically to test semiconductor devices. More particularly, the invention relates to such probe stations having EMI shielded enclosures for substantially enclosing the test devices, such as those probe stations shown in commonly-owned U.S. Pat. Nos. 5,266,889 and 5,457,398 which are hereby incorporated by reference.
The probe stations shown in the foregoing patents are capable of performing both low-current and high-frequency measurements within a single shielded enclosure. However, as electrical test currents decrease, or as electrical test frequencies increase, the use of merely a single EMI shielding enclosure becomes less adequate. In the most sensitive of measurements, and particularly (although not necessarily) when guarding is employed for low current measurements as described in U.S. Pat. No. 5,457,398, the choice of the shield potential is critical. Reflecting such criticality, the single shield enclosures shown in the foregoing patents have in the past been equipped with selective connectors enabling the shield potential to match that of the measurement instrumentation ground while being isolated from other connectors, or alternatively to be biased by another connector, or to be connected to AC earth ground. Usually the measurement instrumentation ground is preferred since it provides a “quiet” shield ideally having no electrical noise relative to the measurement instrument. However, if the shielding enclosure is exposed to EMI (such as electrostatic noise currents from its external environment), its ideal “quiet” condition is not achieved, resulting in unwanted spurious currents in the chuck assembly guard element and/or the supporting element for the test device. The effect of such currents is particularly harmful to the operation of the guard element, where the spurious currents-result in guard potential errors causing leakage currents and resultant signal errors in the chuck element which supports the test device.
For high-frequency measurements, guarding is typically not employed. However, for the most sensitive of measurements, the “quietness” of the shield is still critical. For this reason, it is common practice to construct a fully shielded room, commonly known as a screen room, large enough to contain a probe station with its own separate shield enclosure, test equipment, and several operators. However, screen rooms take up a large amount of space, are expensive to build, and are ineffective with respect to noise sources within the room.
The environmental influences which ordinarily compromise the desired quiet condition of a shield are the motion of external objects at constant potential which cause spurious shield currents due to varying capacitance, and external AC voltages which cause spurious shield currents via constant capacitance. For sensitive measurements, what is needed is a truly quiet shield unaffected by such environmental influences.
Also, to reduce the need for a screen room, and provide a shield unaffected by closely adjacent environmental influences, such quiet shield structure should be compact.